1. Field of the Invention
The present invention relates to a reflection type projector using a reflection type image forming device and, more particularly, to a reflection type projector in which three colors used for representation of a color image are overlapped so that brightness of the light projected onto a screen increases.
2. Description of the Related Art
In general, a reflection type projector is for providing an image by projecting an image generated by a reflection type image forming device onto a screen using an additional light source.
A reflection type projector for representing a color image by simultaneously projecting three colors to improve the brightness of image suggested by the present applicant is disclosed in U.S. patent application Ser. No. 09/057,585 (filed on Apr. 9, 1998, entitled "Refection Type Projector" by Soon-cheol Choi). The suggested reflection type projector, as shown in FIG. 1, includes a light source 110 for emitting light, reflecting mirrors 132, 133 and 134, first and second dichromatic mirrors 131 and 135 arranged on an optical path for transmitting or reflecting input light according to the wavelength thereof, first, second and third optical path converting devices 140, 150 and 160 for converting a proceeding path of input light, first, second and third image forming devices 145, 155 and 165 for forming an image from input light, a dichromatic beam splitter 170 for allowing the input light having passed the first, second and third optical path converting devices 140, 150 and 160 to proceed in one direction, and a projection lens unit 180 for magnifying and projecting the light input from the dichromatic beam splitter 170.
The first, second and third optical path converting devices 140, 150 and 160 each include first through third converging lenses 141, 151 and 161 and first through third prisms 143, 153 and 163. Here, to have light input at an angle greater or less than the critical angle, the first through third prisms 143, 153 and 163 include first through third transmission/reflection surfaces 143a, 153a and 163a, respectively, which transmit the light input from the light source 110 and total-reflect the light reflected by the first through third image forming devices 145, 155 and 165.
The dichromatic beam splitter 170 has three incident surfaces 171, 172 and 173 and a single exit surface 174. The three incident surfaces 171, 172 and 173 face the first, second and third optical path converting devices 140, 150 and 160, respectively, and convert a proceeding path of input light having passed each of the first, second and third optical path converting devices 140, 150 and 160 to proceed toward the single exit surface 174. For this, the dichromatic beam splitter 170 has first and second mirror surfaces 176 and 177 for selectively transmitting or reflecting the input light according to the wavelength thereof. The first mirror surface 176 is coated for the transmission or reflection of the input light according to the wavelength thereof so that the input light passing the first optical path converting device 140 is reflected and the input lights passing the second and third optical path converting devices 150 and 160 are transmitted. The second mirror surface 177 is coated for the transmission or reflection of the input light according to the wavelength thereof so that the input lights passing the first and second optical path converting devices 140 and 150 are transmitted and the input light passing the third optical path converting device 160 is reflected. Thus, as the dichromatic beam splitter 170 includes the first and second mirror surfaces 176 and 177, the lights input from three incident surfaces 171, 172 and 173 are output through the single exit surface 174.
The projection lens unit 180 is arranged between the dichromatic beam splitter 170 and a screen (not shown) for magnifying and projecting the light input from the dichromatic beam splitter 170 toward the screen.
The reflection type projector having the above structure, as shown in FIG. 2, is realized into an off-axis optical system in which an illumination optical axis A.sub.L and an optical axis A.sub.P of the projection lens unit are different from each other. Accordingly, the first through third prisms 143, 153 and 163 and the dichromatic beam splitter 170 have such an optical arrangement as that shown in FIG. 2.
Here, when the illumination optical axis A.sub.L crosses a definite line where the first and second mirror surfaces 176 and 177 cross, the efficiency of use of three colors is improved. However, in the above arrangement, as a first FLCD (ferroelectric liquid crystal display)147 of the first image forming device 145 and a second FLCD 157 of the second image forming device are arranged too closely, installation of convergence adjustment devices 149 and 159 for adjusting convergence of the first and second FLCDs 147 and 157 becomes difficult. The reference numeral 167 denotes a third FLCD. Also, as the light input to the three incident surfaces 171, 172 and 173 of the dichromatic beam splitter 170 after passing the first through third prisms 143, 153 and 163 do not make an angle of 90.degree., the optical arrangement becomes complicated and the first through third prisms 143, 153 and 163 occupy a large space.